Geometry of superimposed colinear deformations in the Ikerasak area, Umanak district, central West Greenland

Abstract

In the Rinkian belt of West Greenland, Proterozoic sediments were deposited on a late Archaean basement of tonalitic orthogneisses which contained thin supracrustal horizons. These rocks were interleaved by deformation in the Early Proterozoic, when Proterozoic structures may have been superimposed on Archaean structures in the gneisses. In the Ikerasak area, because no Proterozoic sediments have been recognised, the age of structures is uncertain and this underlines the need for structural correlation with adjacent areas containing Proterozoic supracrustals. In the Ikerasak area most supracrustals have had a longer deformation history than the surrounding gneisses and intrusive contacts with the gneisses are preserved locally. Such supracrustals are therefore Archaean. Intrusive contacts were mostly obscured by a deformation during which banded gneisses formed. This banding was further attenuated and large recumbent isoclinal folds formed during a deformation which occurred under upper amphibolite facies conditions (D p). The folds may have a sheath-like geometry, and axes plunge gently parallel to a WNW trending extension lineation. Almost coplanar and colinear reworking in lensoid domains of high strain followed (D r). The tonalitic gneisses with supracrustals comprise the main gneiss association and underlie tonalitic to dioritic orthogneisses of the Talerua association in which D p and D r folds are absent. Both associations were affected by a later deformation, D s. In the Talerua association a gneissic banding was folded during D s. This banding formed during superposition of the associations at the same time as the banding in the main gneiss association. Absence of D p and D r folds in the Talerua association may indicate that the boundary between the associations was a deformation boundary when these structures formed in the main gneiss association. This boundary was reworked by the Ikerasak shear zone during D s. Rocks overlying the shear zone were displaced westwards. Recrystallisation occurred at lower grade within the amphibolite facies than during D p and D r. Below the shear zone D s strain soon dies out, but above, in the Talerua association, penetrative D s fabrics are widespread. Flat-lying structures steepen southwards across the study area into the Talerua shear zone. This WNW trending, steep, D t structure had a subhorizontal movement direction, and developed under lower temperature conditions than D s, but still within the amphibolite facies. In central Ikerasak D s fold axes are parallel to the earlier linear fabrics and lie within a few degrees of the extension lineation within the Ikerasak shear zone. South of the shear zone, D s and D t fold axes, and the extension lineation in strongly deformed rocks in the Talerua shear zone, are all parallel. This remarkable degree of colinearity is analysed. On the north side of the Ikerasak shear zone, D s folds are buckle folds formed near the termination of the shear zone. Their orientation is consistent with initiation almost parallel to the extension lineation, in the trailing edge domain of a shear zone in which the flow was a combination of progressive simple shear and irrotational deformation. On the south side of the shear zone, initial buckle folds formed in the complementary leading edge domain and were rotated to the D s extension lineation and amplified passively during progressive D s deformation. In the Talerua shear zone, flow by progressive simple shear, modified by components of irrotational deformation, cannot account for D t fold orientation. The extra factor responsible for the parallelism of D t with D s fold axes, irrespective of D t strain intensity, may be bending anisotropy induced by D s linear fabrics which exercised an important control on the orientation of D t fold axes.